Scroll by page height when list items span multiple pages

[Copilot]

When a list item is taller than the viewport (e.g., long chat responses), list.focusPageUp/Down now scrolls within that item instead of immediately jumping to the next/previous item.

Changes

Modified focusNextPage and focusPreviousPage in listWidget.ts:

• Detect when focused element extends beyond viewport bounds
• Scroll by viewport height while maintaining focus on the same item
• Move to next/previous item only after reaching element boundaries
• Preserve existing behavior for normal-sized items

Example scenario:

Item height: 500px, Viewport: 200px
User on Item 0, scrollTop: 0

PageDown #1: scrollTop → 200px (still Item 0)
PageDown #2: scrollTop → 400px (still Item 0)
PageDown #3: Focus moves to Item 1

Added test coverage:

• Tall items spanning 2.5 pages
• Multiple page-down operations within single item
• Page-up scrolling back through tall item

Original prompt

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This section details on the original issue you should resolve

<issue_title>Scroll lists by page</issue_title>
<issue_description>We have the commands list.focusPageUp/Down for paging in lists, but this only works by moving focus to the index that is up/down a page, and doesn't work right for long list items. We also have list.scrollUp/Down to just scroll the list, but it's slow. I think it would be helpful, especially for chat where list items are really long, to have a paging command that can just scroll. microsoft/vscode-copilot-release#1482 (comment)

I think it would make sense for list.focusPageUp/Down to also just scroll, when the same row takes up multiple pages. Could also add a new scroll command for this though. cc @joaomoreno </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Scroll lists by page #226194

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